A presser bar in a sewing machine transmits pressure to a presser foot at one end of the bar, and the foot transmits this pressure to the work, or material to be sewn. At times during the sewing operation, for example when one stitching operation has been finished and the work is being removed to be replaced by new material, the presser bar is moved longitudinally away (which means normally upward) from the work by means of a lever. When the operator desires to reapply pressure to the work or to begin sewing on new work, the lever is actuated in the reverse direction, and force is transmitted from a biasing spring to move the presser bar longitudinally into engagement with the work then in position to be sewn.
It is desirable that the force of the spring act as nearly precisely axially along the presser bar as is possible without exerting any side forces that would tend to pivot the presser bar about some axis more or less perpendicular to the longitudinal direction of the bar.
One way of applying pressure in the desired longitudinal direction is to place a compression spring between the upper end of the presser bar and a fixed member. Unless the diameter of the spring is large relative its length, a compression spring has a tendency to bend, displacing its central coils from alignment with coils at the ends. In a sewing machine, there is not enough space to have a very large diameter spring supply the force for the presser bar.
It is also desirable to utilize a compression spring that is relatively long, but this only exacerbates the lateral displacement of central coils of the spring. In order to limit such displacement, it has been the practice to enclose the compression spring in a hollow end of the presser bar of slightly larger inside diameter than the outside diameter of the spring. A cylinder extending into the mouth of the hollow end holds the spring in place. Although this does not entirely prevent the undesired lateral displacement, it limits such movement of the central coils and it also prevents the spring from becoming completely disengaged from either the end of the presser bar or the fixed member. However, the spring can still flex laterally enough to rub on the inner wall of the hollow tubular member, which is undesirable. In addition, the presser bar can also engage the outer surface of the cylinder due to some sidewise pressure applied to the presser foot. These limitations on the satisfactory movement of the spring and the pressor bar result in a hysteresis effect, which is also known as "stick slip".
The use of an extension spring to apply downward force to a presser bar to bias it against the work in a sewing machine has been suggested by Rodman in U.S. Pat. No. 823,442, by Feigel in U.S. Pat. No. 1,749,529, by Niekrawietz in U.S. Pat. No. 3,282,237, and by Giesselmann et al in U.S. Pat. No. 4,044,701. However, in each of those patents the force of the extension spring was not applied directly along the axis of the presser bar but was applied to one side of the axis, thereby producing a mechanical moment resulting in hysteresis.